Electrically insulated switching element drive circuit

Abstract

An electrically insulated type of switching element drive circuit supplies an AC voltage via a transformer to a control electrode drive circuit that controls a switching element, with the frequency or amplitude of the AC voltage being modulated in accordance with required on/off operation of the switching element, and with the output AC voltage from the transformer being rectified to obtain a DC supply voltage that is continuously supplied to the control electrode drive circuit. The control electrode drive circuit detects a condition of the AC voltage and controls the switching element accordingly.

1. An electrically insulated type of switching element drive circuit, for controlling on/off operation of a switching element in accordance with an externally supplied control command signal, said switching element drive circuit comprising:
 a control electrode drive circuit for supplying a drive signal to a control electrode of said switching element, to control said on/off operation based on said control command signal; 
 a power supply circuit for supplying electric power to operate said control electrode drive circuit; 
 a transformer for supplying an AC voltage to said power supply circuit from a secondary side of said transformer, said power supply circuit performing rectification and smoothing of said AC voltage from said secondary side of said transformer to thereby obtain a DC voltage, and supplying said DC voltage to operate said control electrode drive circuit; and 
 an AC voltage output circuit for supplying said AC voltage to a primary side of said transformer; 
 a power transmission system that extends from said AC voltage output circuit to said power supply circuit comprises a resonant circuit, 
 wherein:
 said control electrode drive circuit comprises means for detecting a condition of said AC voltage relating to a frequency of said AC voltage and for altering a magnitude of a drive voltage applied to said control electrode, based upon results of said detection of said condition of said AC voltage, 
 said AC voltage output circuit is responsive to an on status of said control command signal, indicating that said switching element is to be set in said on state, for establishing a frequency of said AC voltage that is substantially identical to a resonance frequency of said resonant circuit, and 
 said AC voltage output circuit is responsive to an off status of said control command signal, indicating that said switching element is to be set in said off state, for establishing a frequency of said AC voltage that differs from said resonance frequency by a predetermined amount. 
 
 
   
   
     2. An electrically insulated type of switching element drive circuit according to  claim 1 , wherein said transformer comprises a coreless transformer, further comprising at least one capacitor coupled to said transformer for functioning in conjunction with an inductance of said transformer to constitute said resonant circuit. 
   
   
     3. An electrically insulated type of switching element drive circuit according to  claim 2 , wherein said transformer configured as a coreless transistor comprises
 a circuit substrate formed of an electrically insulating material, 
 a primary coil formed as a pattern of electrically conductive material on a first face of said circuit substrate, having a spiral configuration, and 
 a secondary coil formed as a pattern of electrically conductive material on a second face of said circuit substrate, opposite to said first face, having a spiral configuration corresponding to said spiral configuration of said primary coil, with each part of said pattern of the secondary coil being located substantially directly opposite a corresponding part of said pattern of the primary coil. 
 
   
   
     4. An electrically insulated type of switching element drive circuit according to  claim 3  wherein said circuit substrate is a printed circuit board and wherein each of said primary coil and secondary coil is formed as a printed pattern of a metallic material, with said patterns formed on opposing main faces of said printed circuit board. 
   
   
     5. An electrically insulated type of switching element drive circuit according to  claim 3  wherein at least 80% of said pattern constituting said primary coil coincides in position with said pattern constituting said secondary coil, with respect to a direction at right angles to said first and second faces of said circuit substrate.